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Rats

In August 1903 eminent naturalist Henry Travers wrote to the Colonial Secretary Sir Joseph Ward about “Reserves for Scenery and Native Birds, etc”. Travers had seen first-hand the formerly abundant birdlife on Takapourewa (Stephens Island). But “a year or so later cats were introduced; I was there a few weeks ago, and there is not a native land bird left on the island in consequence of the depredation of the cats”. Of course the loss of birds included the extinction of the Stephens Island wren.

Takapourewa is in the middle distance to the north of Puangiangi and the Rangitoto group. Henry Travers went on to say “I also venture to suggest, that a number of the Tuatara lizards (from Takapourewa) be placed on the two Rangitoto islets on which there is bush; this would prevent any possible chance of their becoming extinct in this portion of the Colony”.

Travers showed insight into predator-prey interactions at the turn of the 20th Century or earlier, and advocated removal of pigs, goats and cats from Kapiti before it was used as a bird sanctuary. His suggestion for the Rangitotos to be a tuatara sanctuary might have been forlorn though, as they were certainly already carrying rats.

Rodents and mustelids and possums took a long time to be recognised as pre-eminent causes of decline and extinction of New Zealand’s native birds, lizards and invertebrates. Elsewhere in the world habitat loss is often key. There, birds and mammals for example have co-existed for millions of years and the birds have evolved to cope with mammalian predation. Not so on our small piece of Gondwana, yet even today those only peripherally interested in conservation, and those with agendas that it suits, stick to the notion that things will be all right if only the forests are protected from destruction.

Ninety-five years after Travers speculated that the Rangitotos might become a sanctuary for tuatara, Nicky Nelson from Victoria University and Peter Gaze were on Takapourewa. Nicky was doing a PhD on the induction of eggs from gravid tuatara and their incubation back in the lab, to better understand the role of temperature in determining the sex of the hatchlings.

Nicky and Peter discussed how the 400 young tuatara resulting from Nicky’s work could best be used to establish a new wild population. She pointed (quite possibly from the same spot as Henry Travers) to Wakaterepapanui, the northernmost of the Rangitotos some 10 km to the south, remarking on what ideal habitat it would be. The problem was that Ecology Division of DSIR had found that Norway rats were present on all three islands and Tim Markwell (Victoria University) had more recently confirmed the presence of kiore on Wakaterepapanui. These rats would prey on tuatara eggs and young as well as out-competing them for invertebrate food.

Norway rats and kiore do not co-exist easily and it is tempting to think that kiore were once widespread on all three islands, but that Norway rats were introduced to Tinui after European colonisation and gradually swam to the northern islands, replacing kiore in the process. Were the few surviving kiore on Wakaterepapanui evidence of more recent colonisation there by the Norway rat? Possums and ship rats were fortunately absent, and there have been only two records of single stoats from Puangiangi. Stoats, present on D’Urville and within theoretical swimming distance, are efficient predators of Norway rats, and the presence of the latter therefore indicated that stoat invasion was rare and that they had not bred on the islands.

If the potential of Wakaterepapanui to house tuatara was to be realised then the rats had to go, and from all three islands to prevent re-invasion one to the other. This was feasible given the wins already had with aerial brodifacoum by the late 90s, but it was relatively expensive. The consent of three different landowners would also be needed. Wakaterepapanui is publically owned and managed by the Department of Conservation. Puangiangi in the middle was owned and occupied by Ross Webber and the southernmost, Tinui and its islet Takawhero, had a multitude of Maori owners but was by then uninhabited.

Victoria University took the initiative and got a grant from the South Pacific Conservation and Development Fund, which used the money obtained from the French government after they had bombed the Rainbow Warrior and killed a crewman. With finances covered it now fell to Peter and his colleagues at DOC to obtain consent for the operation. Discussions with many of the individual owners of Tinui went well– the eradication of rats being consistent with a management plan previously prepared by Tony Whitaker for the owners of the island. It was not possible to meet with every owner but a senior kaumatua of Ngati Koata deemed that there was sufficient endorsement. Ross Webber was also keen to see the end of rats on his island. He had been trapping and shooting them, without measurable effect, and he was sick and tired of them getting into his stores. A complication was that the aerial application of brodifacoum was licensed for stock-free islands only and Ross still had a good flock of romneys on Puangiangi.

By early July 1999 a 50-page Assessment of Environmental Effects had been prepared which resulted in DOC approval for the operation, the Medical Officer of Health had been notified and the Marlborough District Council had given a non-notified resource consent. The bait was ordered. In August all of Ross’ sheep were barged to a neighbouring farm on D’Urville Island. This was not without some difficulty despite the services of shepherd Lionel Roughton and his dogs: these sheep just weren’t used to being rounded up with dogs, Ross’ last one having died some time previously. As the sheep needed to stay off the island until all bait had degraded, Peter arranged with the Rai Valley butcher to get a meat pack for Ross onto the weekly mail boat.

Ross Webber’s sheep head for D’Urville August 1999. Peter encourages lead sheep on board at the end of a sturdy rope.

After these months of preparation the actual operation, organised by Mike Aviss and DOC staff from Picton, was completed in a day – 24 August 1999. There was no non-toxic pre-feed. The bait [Pestoff 20R as 6 g pellets with a concentration of 0.02 g/kg brodifacoum] was flown from a barge moored at Tinui and applied in sixty-metre-wide swathes at 15 kg/ha. Bait was spread by hand on Takawhero to the southeast of Tinui.

Ready to spread the brodifacoum, 24 August 1999

There is good reason to believe the eradication was successful. Trapping soon after the operation failed to detect rats on any of the islands. The juvenile tuatara were then duly moved to Wakaterepapanui, along with green gecko and giant weta.

The helicopter finds a place to land on Wakaterepapanui; tuatara being unloaded. Photo courtesy Jo Monks.

When Puangiangi came on the market in 2012, we were not sure it had stayed free of stoats and rats, the two most likely invaders. We also didn’t know about the other islands as Wakaterepapanui is seldom visited and the bait stations on Tinui are not checked. Sue and I were really only considering Puangiangi amongst competing options for our next project because of the predator eradication, and I guess we would have walked away if our pre-purchase visit had shown they were back. Sue remarked that it really is just a useless, though beautiful, piece of rock if it can’t be kept all but free of pests.

When we landed that March, the proxy tracking tunnel on the firm golden sand at Boatshed Bay was free of little mammalian footprints and there was no sign around the buildings (rats would want for shelter more than they would want for food on a place like Puangiangi). Common gecko, which Peter remembered in only small numbers during the 1980s, were everywhere; darkling beetles and giant tree weta (at instars you just don’t see on the mainland) likewise. Our first night in the house in May was frightening though. The scratching and thumping of animals in the walls and ceiling made us think for a while that it must be riddled with rats, until we found it was a favourite gecko residence. Indeed when we opened up some walls for plumbing fittings, 200 mm of accumulated shed gecko skins and droppings were piled up on many of the dwangs. And there were no mummified rats anywhere.

In the future, stoats may occasionally swim from D’Urville, but we hope the swift currents will mean that any arrivals are single (and not pregnant). Any re-invasion of rats would likely be from boats. It was a calculated risk to install moorings on safety grounds, and we can only hope that no-one runs a line to the land when they are moored. There will never be a jetty and we will continue to wade ashore. Trespassers do beach boats and kayaks and this creates considerable risk. The No Landing signs should make it obvious what is required and locals have been asked to watch for illegal landings and to intervene as required. Dogs being let ashore from boats would also be a disaster as they would find some of the ground birds there irresistible.

After the project got under way in 2012, Peter set out a line of DOC200 traps at landing sites and in outbuildings, baited with Erayz and checked monthly. We have since extended the network to 17, with good coverage along the ridgelines and tracks which the animals would prefer to use. If we were never to do anything else on the island, keeping the traps intact and serviced would be the job that does get done.